Drop to gdb on unix when pdb.set_trace is encountered

Merged Bitbucket Importer requested to merge bitbucket/merged-pr-327 into branch/default

Created originally on Bitbucket by timfel

Was already merged in Bitbucket before import, marked as merged by the import user

This is useful for us in RSqueak, where we define a primitive that calls pdb.set_trace. In hosted mode, this normally drops to pdb, but especially launching an image takes 10min+ in hosted mode, so we also like to debug using gdb in translated mode. This just makes it easier

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